Calcite Veins in Black Rock Limestone

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Crystals in limestone rock cracks

On the beach below the Coast Guard Lookout, at the tip of the Rhossili Headland in Gower, jagged rocks stick up in lines parallel to the shore. This is the landward edge of the wave-cut platform that forms the causeway between the mainland and the Worms Head. The causeway is covered by the sea twice a day. Here on the beach, the rocks have not been worn down so much as the rest of the causeway, and the repeating rows of strata with their pronounced bedding planes are clear to see. The rock layers comprise Black Rock Limestone which was formed in the Carboniferous Period.

At some time in the past, cracks opened up in the rocks, breaking it at right angles to the rock layers. These fissures became filled up with calcium from solution in ground water that precipitated out as large calcite crystals on the sides of the spaces. Red sediments containing iron, haematite, also percolated down from the former covering of Triassic Period rocks, infilling the gaps between the lining of calcite crystals in the cracks. Today, the once hidden rocks are exposed to the surface in the base of the anticline that once connected the mainland to Worms Head, and the red and white stripes of calcite and haematite are visible cutting across the grey limestone rocks on the shore.


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